The membrane bioreactor (MBR) unit combines two basic processes: biological degradation and membrane separation-into a single process where suspended solids and microorganisms responsible for biodegradation are separated from the treated water by a membrane filtration unit. The entire biomass is confined within the system, providing for both control of the residence time for the microorganisms in the reactor (mixed liquor age) and the disinfection of the effluent.
In general, influent enters the bioreactor, where it is brought into contact with the biomass.
The mixture is filtered through the membrane using a pump, water pressure or a combination of both. The permeate is discharged from the system while the entire biomass is maintained in the bioreactor The permeate is discharged from the system while the entire biomass is returned to the bioreactor. Excess mixed liquor is pumped out in order to maintain a constant mixed liquor age, and the membrane is regularly cleaned by backwashing, chemical washing, or both.
Membranes used in the MBR unit include ultra- and microfiltration, inner and outer skin, hollow fiber, tubular, and flat, organic, metallic, ceramic, and the like. Preferred membranes for commercial application include hollow fiber with an outer skin ultrafilter, flat sheet ultrafilter and hollow fiber with an outer skin microfilter. Preferred membrane pore size is 0.01-5 micron.
In the aerobic membrane bioreactor (MBR) process, membrane fouling has always been a significant issue limiting the hydraulic performance of the process. Due to membrane fouling, MBR throughput or flux often decreases and more membranes are required to compensate for the throughput loss.
Recently, many research results (Nagaoka et al, 1996, 1998; Lee et al., 2002) have shown that one of the main causes of membrane fouling is biopolymers, which includes polysaccharides and proteins secreted by the biomass present in the mixed liquor of the MBR. In addition, a number of inorganic scales formed in bioreactors have been reported, where the salt concentrations in the influent were relatively high. As a result of scale formation on the membrane surface, the membrane performance was significantly reduced (Huisman, 2005; Ognier, 2004).
To prevent membrane fouling caused by biopolymers, methods were developed using cationic polymers that do not react with negatively charged membranes in contact with the mixed liquor (Collins and Salmen, 2004). In this method, various polymers are added directly to the aerobic MBR usually to the aeration tank and these polymers react with the biopolymers. The resulting particles, which consist of biopolymers and polymers, have considerably lower membrane fouling tendencies.
The same microbiologically produced polysaccharide and protein biopolymers produced in MBRs that cause membrane fouling are also known to cause foaming in the MBR mixed liquor. This is because these compounds contains many surface active functional groups that help stabilize foam at the air-water interface. In addition, MBRs often contain significant amounts of filamentous microorganisms that have been correlated to foam formation. Both the biopolymers and filamentous microorganisms react with the cationic polymers described in this invention. Previous work has shown foam reduction or foam elimination always occurs at the same time that cationic polymer has been observed to improve membrane flux. (Richard, 2003).
In the mean time, anoxic and anaerobic tanks are increasingly being installed in MBRs to increase nitrogen and phosphorus removal efficiencies. In these conditions, the aerobic biomass will be periodically exposed to oxygen scarce conditions while the anaerobic biomass will be exposed to aerobic conditions, since the mixed liquors are recycled between oxygen rich and oxygen scarce conditions. Therefore biomass will produce more biopolymer due to oxygen stress. Apart from the accelerated biopolymer generation triggered by the cyclic oxygen concentrations, biopolymer generation also can be accelerated by low dissolved oxygen (DO) conditions in anoxic and anaerobic tanks (Calvo et al., 2001).
The most direct evidence of the accelerated membrane fouling at low DO situations was obtained in Kang et al.'s experiment (2003). In their experiment, nitrogen gas was used to continuously scour the submerged membranes, while air was supplied through separate nozzles to the area above which no membrane was placed. The permeate flow was constantly maintained at 20 L/m2/hr. As soon as air supply was stopped, TMP started to increase and DO started to decrease.
Accordingly, if anoxic and/or anaerobic tanks are installed in a MBR process, the biopolymer content in the mixed liquor will be higher than that in other MBRs having only aeration tanks. Therefore, if the MBR contains anoxic and anaerobic reactors, the previous method (John et al, 2004) will be considerably less effective in terms of dosage and flux improvement. In addition, the previous method would not be effective in anaerobic MBRs, which includes anaerobic digester as a sole bioreactor or one of the bioreactors. A more effective and economic method, which allows better performance and lower dosage, is necessary.
Apart from the biopolymer problem, recently, inorganic fouling has been reported in a number of MBRs (Huisman, 2005; Ognier et al, 2002). This inorganic fouling often consists mainly of calcium carbonate (CaCO3) and/or calcium phosphate, which may precipitate in the aerated biological wastewater treatment or directly onto the membrane (“scaling”). The inorganic fouling also includes iron oxides.
Aeration in the treatment tank (and in the membrane tank) can lead to inorganic fouling by various routes. For example, aeration drives the dissolved CO2 out of the wastewater and this pushes the equilibrium of reaction (1) to the right.HCO3−CO32−+CO2 (g)   (1)
The carbonate (CO32−) formed by reaction (1) precipitates with calcium that is present in the wastewater to form CaCO3 (limestone). Moreover, reaction (1) will cause an increase in pH, which will favor calcium phosphate and iron oxide precipitation. The precipitation of carbonates and phosphates will partly take place in the bulk wastewater and this will form small particles, of which most will be retained by the membranes. This precipitation will also take place on all surfaces, among which is the membrane surface.